Remove unsampling logic from agent
Remove unsampling logic from agent

Unsampling has been moved into V8 with
https://codereview.chromium.org/1706343002 so this logic is no longer
necessary in the agent.

This change also updates the units used for emitted profiles to be more
consistent with other profilers.

google/pprof-nodejs,google/pprof-nodejs,googleapis/cloud-profiler-nodejs,google/pprof-nodejs,googleapis/cloud-profiler-nodejs,google/pprof-nodejs,matthewloring/cloud-profiler-nodejs,googleapis/cloud-profiler-nodejs,googleapis/cloud-profiler-nodejs,matthewloring/cloud-profiler-nodejs,matthewloring/cloud-profiler-nodejs,googleapis/cloud-profiler-nodejs,google/pprof-nodejs